1. Technical Field
This invention relates generally to spiral antennas and, more particularly, to a multi-arm spiral antenna that is capable of providing both right and left hand circular polarized energy for a large number of operating modes.
2. Discussion
Spiral antennas are generally well known for providing constant directivity gain, beamwidth and impedance over broad frequency ranges and are particularly advantageous for operating at circular polarization. In the past, spiral antennas were commonly employed to transmit and/or receive electromagnetic energy having either a right-hand circular polarization or a left-hand circular polarization, in addition to detecting some linear polarized energy. For instance, a conventional center-fed multi-arm spiral antenna generally included conductive spiral arms wound in the counterclockwise direction which in turn would detect right-hand circular polarization, while spiral antennas having spiral arms wound in the clockwise direction would generally detect left-hand circular polarization. However, such single sense circular polarized spiral antennas generally are not capable of detecting both right-hand and left-hand circular polarization and therefore remain blind to electromagnetic waves of the opposite circular polarization.
One solution for achieving dual circular polarization operation is to provide separate left-hand circular polarization and right-hand circular polarization spiral antennas. However, the use of two separate oppositely polarized spiral antennas generally would amount to duplicate antenna components which in turn leads to increased cost and additional space requirements.
Another technique that has been employed to configure spiral antennas for operation with both right-hand and left-hand circular polarization is described in U.S. Pat. No. 3,681,772 issued to Ingerson. The aforementioned patent issued to Ingerson is incorporated herein by reference. According to the Ingerson approach, a center-fed modulated arm width spiral antenna is provided which comprises a series of cells formed by one section of antenna arm having a first relatively narrow width dimension followed by a second section of antenna arm of substantially greater width dimension. These cell sections form arm width modulations which are positioned along the antenna arms to establish impedance discontinuities or reflection regions which are intended to selectively reflect the outwardly flowing currents to produce reflected currents corresponding to the opposite sense of circular polarization. Thus, operation is achieved for both left-hand and right-hand circular polarization by establishing the proper arm width modulations.
However, the modulation approach described in U.S. Pat. No. 3,681,772 discloses a four-arm spiral antenna for providing up to two or possibly three modes of operation. Currently, there is an increasing need to achieve a larger number of modes of operation with spiral antennas which would provide increased accuracy angle-of-arrival information. An increased number of modes would advantageously increase the gain at small elevation angles near the horizon (i.e., plane of the antenna). This increased gain is generally due to the high order mode capability which effectively concentrates radiation close to the horizon. A higher-order multi-mode spiral could therefore be used to advantageously offset gain degradation that may otherwise be associated with the particular location of the spiral antenna on an aircraft that may involve a compromise in the radiation patterns and gain exhibited thereby.
It is therefore desirable to provide for a multi-arm spiral antenna which is capable of providing simultaneous dual circular polarization with a relatively high number of operating modes. It is further desirable to provide for such a center-fed dual circular polarization spiral antenna which has at least eight spiral antenna arms for providing at least six modes of operation. In addition, it is desirable to provide for a multi-arm spiral antenna which advantageously exhibits enhanced beam radiation patterns for achieving increased angle-of-arrival accuracy. Furthermore, it is also desirable to provide for such a spiral antenna which exhibits a broad frequency band and offers the convenience of a single antenna package.